Development of the noise theoretical formula for analyzing noise components on CT images.

We established a theoretical framework to analyze the individual noise components on computed tomography (CT) images and evaluated the relationship between CT doses and each component. The model was developed based on CT numbers and the relative noise standard deviation (SD) measured directly on the CT images, enabling noise decomposition solely from image-domain characteristics without relying on raw projection data. CT images of a quality control phantom were obtained at various CT doses using two CT scanners. We measured mean CT numbers and the relative noise SD at each rod position on the CT images and estimated each noise component using the proposed framework. The relative noise SD was expressed as a function of the transmitted dose corresponding to the number of photons through the phantom, where the respective terms in the formula represent the contributions of structural, quantum, and electronic noise components. Results showed that the proportion of quantum noise exceeded 70% at CT doses routinely used in clinical settings, remaining the dominant component. However, at lower doses (CTDIvol ≤1 mGy), the contribution of electronic noise became substantial, reaching approximately 10-30%. This framework can serve as a baseline for future CT dose optimization and provide useful insights for developing image reconstruction and processing methods to effectively reduce image noise.